How long does it take to beat this game?

[aragod]

Barozi said:

So I've completed Assassin's Creed 2 !
I know I have the game only since yesterday, but it's so awesome that I simply had to skip school (lol)
Took me 12 hours to beat the main story. Overall 60% completion and got 700G on the way. So there's still a lot of things to do !

Were you trying for a speed run or is it really that short? I was expecting a bit longer adventure from an open world game.